1. Field of the Invention
The present invention relates to a method of quite accurately molding plastics and to an injection compression molding machine for molding plastics with high dimensional accuracy.
2. Description of the Prior Art
In recent years, plastics have been molded by injection molding with high dimensional accuracy. For example, it has been required that lenses, optical disk substrates, etc. be molded to tolerances of submicron order.
For molding an article accurately, it has been the common practice to adopt the following process. First, a mold is clamped by applying a high pressure. Then, a molten resin is injected into the cavity of the mold through a nozzle. Immediately after the cavity is filled with the resin, a holding force large enough to prevent the mold from opening is imparted to the mold so that the shape of the molded product becomes as close as possible to the shape of the mold. The molding force is set less than the clamping force. As the specific volume of the molten resin in the mold is made closer to the specific volume of the resin at room temperature by adjusting the pressure and the temperature, sink marks, or shrink marks, which are produced on the molded product during cooling are reduced. In practice, however, a very large force is needed to satisfy such requirement. Under high pressure conditions, the mold and the construction elements of the molding machine including tie bars must be sufficiently rigid to prevent them from deforming. For this purpose, the mold and the molding machine are rendered bulky. This frequently results in a very large molding system.